A practically important pigment, which shows bright color tone and has high coloring power, generally is in the form of very fine particles. If the particle size is made to be much smaller so as to enhance the coloring power, however, the pigment dispersion liquid often exhibits such high viscosity that some troubles are apt to be caused in its industrial production process, that is, the dispersion liquid may hardly flow out of a dispersing apparatus, and it sometimes is impossible to transport the dispersion liquid through pipe lines. Further, the pigment dispersion liquid having such high viscosity may turn into gel in the course of storage.
A colored photosensitive composition containing the pigment is useful, for instance, as an image-forming material for producing color proofs or color filters. The pigment contained in the colored photosensitive composition is required to exhibit bright color tone and to show high coloring power.
In the process for forming a color image using the colored photosensitive composition, the composition is coated on a substrate to form a colored photosensitive layer, and then exposed through a predetermined pattern and developed. As the developer used in the developing procedure, an aqueous alkaline solution is more frequently employed than an organic solvent in consideration of environmental preservation. Hence, the unexposed photosensitive layer is required to be soluble in the aqueous alkaline solution. On the other hand, an organic solvent is advantageously used as a solvent (i.e., dispersing medium) of the coating liquid containing the colored photosensitive composition because the coating liquid in the organic solvent is easily dried. Therefore, a binder contained in the composition is generally required not only to have an acidic group but also to be soluble in an appropriate organic solvent, and in fact the conventional colored photosensitive composition comprises organic pigment particles dispersed in the binder having an acidic group.
Since the layer of a colored photosensitive composition is generally required to be very thin and to have high coloring power, the organic pigment must be very finely dispersed in the binder (which has an acidic group and further which is soluble in an organic solvent). As described hereinbefore, the colored photosensitive composition (i.e., a pigment dispersed photosensitive composition) is useful, for example, as a material for forming a color filter of a liquid crystal display unit.
The color filter comprises red, green, and blue colorants on a transparent substrate in the positions of respective pixels so that light can selectively pass through the pixels to display the desired color image. In the process for producing the filter, the colorants are placed on the substrate by various methods such as dyeing method, printing method, electrodeposition method, and pigment dispersion method. The pigment dispersion method is most preferred because the production process is reliable and it gives a color filter of high quality.
The pigment dispersion method comprises the steps of coating the transparent substrate with the colored photosensitive liquid composition containing the pigment composition to form a colored photosensitive layer; pattern-wise exposing the layer; developing in a developing solution the layer to form a pixel pattern of the first color, and then repeating those steps for each color to prepare a plural number of color image patterns each consisting of a large number of pixels. Since the colorant is a solid pigment in the form of fine particles (which is insoluble in ordinary solvents such as organic solvents) in this method, the pigment particles may scatter or absorb the applied light to lower the transparency of the filter if the particle size is large. Further, in that case, the axis of polarized light may be rotated by scattering and/or birefringence, and consequently the contrast of an image given by the liquid crystal display unit may lower (ref., "Seventh Color Optics Conference (1990)", a color filter for 10.4" size TFT-LCD display of 512 colors, Ueki, Koseki, Fukunaga and Yamanaka). Hence, the pigment particles are desired to be dispersed finely and uniformly enough and it is requried for the particle size to be appropriately small.
In order to make the pigment particles well dispersed and to give good fluidity to the pigment dispersion liquid, various kinds of dispersants are developed. Generally used dispersants can be roughly classified into two types, namely, a polymer type and a low molecular weight type. Examples of the polymer type dispersants include polyacrylate, sodium maleate-olefin copolymer, polyester having carboxyl terminal (Japanese Patent Publication No. 54-34009), a polyester having an acidic group and/or basic group derived from tetrakis (2-hydroxy-alkyl)ethylenediamine (Japanese Patent Provisional Publication No. H2-245231), and a copolymer derived from the following four kinds of monomer, namely, a macromonomer (i.e., an oligomer having an ethylenic unsaturated group in its terminal position), a monomer having a hydroxyl group, a monomer having a carboxyl group and other monomer (Japanese Patent Provisional Publication No. H8-259876). Examples of the low molecular weight type dispersants include a sorbitan fatty acid ester, a poly-oxyethylenealkylamine, an alkyldiamine and an alkanol amine derivative (U.S. Pat. No. 3,536,510).
The copolymer derived from a macromonomer and others is said to be useful for preparing a colored photosensitive composition for color filter or color proof. Japanese Patent Provisional Publication No. H8-259876 discloses that the above-mentioned copolymer (which is derived from a macromonomer and others) makes the pigment particle size small enough and improves dispersing stability.
The present inventors have examined known colored photosensitive compositions and found that there are the following problems in the known compositions. In the case that organic pigment particles are dispersed in the binder which has an acidic group and is soluble in an organic solvent, the dispersion liquid is apt to show a high viscosity because of certain intermolecular interaction between the acidic group and the organic pigment particle. The viscous dispersion liquid is not preferred in consideration of dispersion of the pigment particles although the dispersion stability is enhanced, and hence more appropriate dispersant is needed so as to control the viscosity and to promote good dispersion of the particles. Among the above-described known lower molecular weight type dispersants, amine compounds are found to be relatively effective to promote the dispersion. However, since the amine compound easily reacts with the acidic group of the binder to form a salt, it sometimes causes some troubles in the developing procedure (for example, the photosensitive resin layer may turns to show less solubility in an aqueous alkaline developing solution). Further, it has been found that the known water-soluble dispersants and various alkaline dispersants have the same problem as the above-mentioned amine compound.